Four megabyte pages on Pentium

Adam J. Richter (
3 Jul 1995 22:46:57 GMT

The front page of the July 3, 1995 issue of _Electronic
Engineering Times_ has an article "Pentium secrets revealed" by
Alexander Wolfe, which claims at a Christian Ludloff, a programmer living
in the former East Germany, has reverse engineered the appendix H
information on programming the pentium and made it available by
anonymous FTP from

According to the article, the registers mostly have to do with
performance monitoring, but there is also support for four megabyte
virtual memory pages, which could be used to improve hit rates on the
translation lookaside buffer when running kernel code by mapping the
the kernel into a single four megabyte page.

On other architectures, big pages are also useful when using a
memory mapped frame buffer, because otherwise TLB reloads can be a
performance factor in graphics operations that span a lot of vertical
screen area, such as drawing vertical or near vertical lines (there
was a paper dumb frame buffer performance at one of the early X
conferences by someone at DEC that discussed this).

However, because of the PC's screwy architecture and because
graphics accelerators have become so cheap and common, the mapping the
kernel is probably the only useful application of four megabyte pages
in linux.

Adam J. Richter				  Yggdrasil Computing, Incorporated
(408) 261-6630				  "Free Software For The Rest of Us."